You've been given a task I wouldn't want to have!
- If a modem or fax isn't set to auto-answer, you'll never find it.
- Nowadays most modems have fax capabilities, so every one of your modem numbers is potentially a fax number.
- IIRC, faxes communicate at either 4800 or 9600 baud, which should sound different to a trained ear than a modern (at least v.32) modem's normal "data" connection.. but who would want have to listen to the screeching of modems handshaking over and over??
The best way to tell if they're fax machines is to try to fax them something! :)